These latter systems print with high resolution, so long as the jets are stimulated by a clean stimulating perturbation; that is, a stimulating disturbance which is a pure sinusoid of the correct frequency and without harmonic distrubances at other frequencies. When the stimulation signal is not clean, the jets produce small satellites as well as primary drops. Furthermore, under more severe conditions of poor stimulation, the print head electrodes may become shorted out, so that printing is interrupted. Reference may be made to Stoneburner U.S. Pat. No. 3,882,508 and to Cha U.S. Pat. No. 4,095,232 for background information on print head stimulation and attendant problems.
Ink jet printers which operate without need of stimulation are disclosed in Hertz et al U.S. Pat. No. 3,416,153 and in Hertz U.S. Pat. No. 3,916,412. The system disclosed in 3,416,153 produces a continuously flowing, non-stimulated jet and controls the jet for printing purposes by applying a high level charge thereto. The high level of charge diffuses the jet to form a spray which is then prevented from reaching the printing surface. Hertz 3,916,421 teaches an improvement of the earlier patent, wherein signal control electrodes are provided to impress an electrical charge directly on the droplets at the drop formation point, and a deflection electrode is provided for deflecting the drops which are so charged. The signal control electrodes are positioned within separate supply tubes which terminate in relatively small nozzles said to have a diameter from 10 to 50 microns.